The present invention relates to a lithium battery, and, more specifically, to an all-solid-state lithium battery that can generate a stable voltage and current.
Lithium batteries have recently undergone remarkable developments in terms of performance improvements, miniaturization, and thickness reduction. These developments have required improvement not only with respect to the material and shape of the negative and positive electrodes, but also with respect to the intervening electrolyte. In particular, solid electrolytes are considered crucial for obtaining a solid-state product, a high flexibility, and a high degree of moldability. These electrolytes, moreover, must have various high-performance attributes such as a high ionic conductivity, high lithium ion transport number, high reliability, and resistance to moisture.
Solid electrolytes comprising lithium salts dissolved and/or dispersed in polymer resin matrices are known in the art. In particular, the synthesis of solid electrolytes that combine polyorganosiloxane chains with the polyethylene oxide (PEO) chains have been actively pursued. For example, Japanese Patent Application Laid-Open [Kokai or Unexamined] Number 62-209169 [209,169/1987] teaches a solid electrolyte obtained by the dispersion of lithium ions in crosslinked material obtained by crosslinking siloxane and PEO using radiation (electron beam and the like) or the Pt-catalyzed hydrosilylation reaction. Japanese Patent Application Laid-Open Number 63-170857 [170,857/1988] teaches a lithium battery that incorporates this solid electrolyte. These methods, however, require the use of organic solvents which are deemed inappropriate for some products and which complicate their manufacturing processes. For instance, the organic solvent can degrade the working environment, damage surrounding substances and persist in the final product. Similarly, the organic solvent can deter the crosslinking reaction by inducing phase separation. This is associated, inter alia, with a deterioration in product quality and a lack of reproducibility.
Japanese Patent Application Laid-Open Number 2-230667 [230,667/1990] discloses a lithium battery which incorporates a solid electrolyte comprising lithium salt dispersed in PEO chain-grafted polystyrene. Although this method does lead to an improvement in material homogeneity, it is disadvantageous in that negative ion transfer occurs due to electrical conduction simultaneous with positive ion transfer. Such negative ion transfer causes polarization within the electrolyte and variation within the structure of the electrode/electrolyte interface with elapsed time resulting in nonsteady-state current flow.
Since compensation for the electrical charge of the positive ion makes the presence of negative ions unavoidable, negative-ion mobility must be minimized as much as possible, i.e., it is desired to have a solid electrolyte whose conduction is based solely on the positive ion (positive ion-monoconductive solid electrolyte). Such electrolytes have been described in the art. For example, Lecture Number 2XIICO8 from the 1988 Annual Spring Meeting of the Chemical Society of Japan describes a lithium ion-monoconductive, sulfonate ion-immobilized solid electrolyte thin film. This film is fabricated by the plasma polymerization of octamethylcyclotetrasiloxane and methyl benzenesulfonate followed by compounding with PEO and treatment with lithium iodide. The complex nature of this plasma polymerization reaction, however, makes production of the target polymer structure highly problematic and, as a result, a perfect lithium ion-monoconductive solid electrolyte is not obtained. Another problem with this method is that the use of plasma polymerization limits this method's range of application.
Thus, even though the lithium batteries proposed to date have incorporated solid electrolytes, they suffer from problems with the properties of the solid electrolytes or with their methods of fabrication.
The present invention takes as its object the production of an all-solid-state lithium battery that generates a stable voltage and current. The present inventors have carried out extensive investigations directed at solving this object and, as a result, discovered that material comprising the dispersion of lithium ion in a particular type of macromolecular crosslinked copolymer is free of the problems listed above and exhibits an excellent ionic conductivity. Moreover, manipulation of the composition makes it possible to obtain a positive ion-monoconductive solid electrolyte material that eliminates the above-listed problems when incorporated into a lithium battery.